The varied uses of wireless devices are increasing, particularly the uses for “smart” devices for residential and commercial building applications. When using these devices, wireless communication of electromagnetic radiation signals is used, which may include radio-frequency (RF) signals.
When used in building applications (e.g., smart homes), one highly desirable feature is maintenance free (battery-less) sensor networks. Such sensors can be coupled with the walls of the building. The sensor may be used for many different purposes, such as to monitor room temperature, air temperature, air quality, humidity, stress on joints, moisture in walls (e.g., bathroom walls) or under shingles, flow of water in pipes, force on the roof (e.g., hail impact or weight of snow), damage due to external forces (e.g., damage from wind), carbon monoxide presence, natural gas and air movement in ducts, among many others. Additionally, these sensors can be powered via various energy harvesting technologies, such as solar energy, vibration energy, etc., as well as ambient RF signals (e.g., AM/FM station signals and television signals).
Powering sensors with dedicated RF signals, such as a carrier wave from a radio-frequency identification (RFID) reader, is a very attractive option. However, the range of typical over-the-air RFID systems is limited by a maximum of a few tenths of feet indoors and is highly dependent on the propagation environment (e.g., a tag just around the corner may not be read). Thus, using a typical system may not provide satisfactory or acceptable performance for many smart building applications.